This really depends on the machine you want to run it on and what you want to do with it. In most cases 64 bit will be faster if you have a 64 bit application to run on it. If you run a 32 bit application it really doesn't make much difference. Most home/office applications are 32 bit so it really doesn't make a difference and that is why most off the shelf systems come with the 32 bit version.
One of the main differences is that 64 bit does not have the 2.75GB memory limit. If we assume you're going to use this for crunching then any project that has 64 bit applications should run faster than 32 bit ones. This is because you are able to process 64 bits at a time compared to 32 bits. One other thing to research before upgrading, are 64 bit drivers available for your hardware?